Shape memory polymers (SMPs) are a class of active materials that can be programmed to “fix” a temporary shape or a series of temporary shapes, and then later to recover to a “memorized” permanent shape established by a covalent or physical network by applying thermal, electrical, or other environmental stimulus. In the case of thermal stimulation, the shape memory polymers are characterized by deforming at a temperature above a softening transition such as glass transition temperature (Tg) and melting point (Tm) of the polymer, then cooling through this transition under stress, causing immobilization of the constituent network chains, and the macroscopic shape to be fixed. Recovery of the permanent shape is then accomplished by heating above the triggering temperature, which re-mobilizes the network chains and allows rubber (entropic) elasticity to return the sample to its equilibrium shape. Depending on the nature of external stimulus, SMPs are categorized as the thermally-induced SMPs, light-induced SMPs, electro-active SMPs, pH-responsive SMPs, and water/moisture-driven SMPs, and various mechanisms are operating in each category. Shape-memory materials, which include shape-memory alloys (SMAs), have been widely used in actuation, aircraft, robotics, piping, medical, and dental applications. It is noted that SMPs differ from SMAs in that their glass transition or melting transition from a hard to a soft phase, which is responsible for driving the shape memory effect, whereas for SMAs, martensitic/austenitic transitions are responsible for the shape memory effect.
There are numerous advantages that make SMPs more attractive than SMAs. For example, SMPs have much higher capacities for elastic deformation (up to 200% in most cases), much lower cost, lower density, a broader range of application temperatures which can be tailored, comparatively easy processing, and potential biocompatibility and biodegradability. However, the state-of-the art SMPs are consisted of high-alkyl content polymers such as, polyurethane, poly(ε-caprolactone), poly(norbornene), (ethylene-oxide)/(ethylene terephthalate)-based copolymers, styrene/butadiene copolymers, thiol-ene/acrylate copolymers, etc. Therefore, none of the foregoing has shape-memory properties temperatures above 150° C. or long-term thermal and thermo-oxidative stabilities at this temperature region.
In recent years, a number of high-temperature shape-memory polymers have appeared in open and patent literature. For example, aromatic polyimides, polyamides, and poly(amide-imide)s are common classes of heat-resistant, thermally stable, polymers with glass transition temperatures in excess of 150° C., and have been crosslinked with multifunctional amine crosslinking agents to impart shape-memory effects, as described in U.S. Pat. Nos. 8,546,614; 8,791,227; and 8,962,890, each of which is incorporated herein by reference in its entirety. However, the multifunctional amine crosslinking agents are limited to polymers bearing amine-reactive groups.
In view of the foregoing, there is a need for new multifunctional crosslinking agents and methods of making and using same to impart shape-memory effects to the parent polymers.